Dilatometric Study of the Formation of Martensite and of the Effects of Stabilization of Austenite in High-Chromium Pipe Steel

Resumo

An analytical method of a dilatometric determination of the dependence of the amount of martensite that arises upon the quenching cooling of austenite in high-chromium pipe steel has been developed. The exponent β in the Koistinen–Marburger equation reaches the usual value of 0.0105 when the γ phase is supercooled by 60 K below M_S. It was concluded that at this stage the formation of martensite is stimulated by the precipitation of carbides from austenite at temperatures of 700–800°C and by the creation of centers of preferential nucleation of martensite. However, with an increase in the rate of cooling the austenite, the stabilization of the γ phase and a decrease in M_S occur. Another variant of stabilization is associated with an interrupted martensitic transformation, and annealing in a two-phase state (200–500°C) followed by cooling, which leads to the appearance of a stabilization step with a width ΔT in the dilatometric curve. The maximum stabilization effect is observed after annealing of short duration (about 10 min) near 400°C.